1. Field of the Invention
The present invention relates to a high-purity vitreous silica crucible used for pulling a single-crystal silicon ingot for semiconductors, and more particularly, to a high-purity vitreous silica crucible with high strength which is capable of preventing generation of deformation, distortion, and the like while suppressing generation of pinhole defects in a single-crystal ingot when used for pulling a large-diameter single-crystal silicon ingot (hereinafter, simply referred to as single-crystal ingot).
Priority is claimed on Japanese Patent Application No. 2007-323420, filed on Dec. 14, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, for a vitreous silica crucible used for pulling a single-crystal ingot, as a raw powder, high-purity vitreous silica powder with an average particle size of 200 to 300 μm and a purity of 99.99% or higher is used. Here, a gap formed between an inner surface of a graphite mold and an outer surface of a core, for example, a gap of 30 mm, is filled with the vitreous silica powder while the graphite mold is rotated at a speed of 60 to 80 rpm (see FIG. 3A). After filling the gap with the vitreous silica powder, the core is removed, and a silica powder compact is then formed. Thereafter, while the graphite mold is rotated at a speed of 50 to 100 rpm, a three-phase AC arc discharger using graphite electrodes is inserted through an upper opening of the silica powder compact. The inside of the graphite mold is heated to a temperature of about 2,000° C. by vertically reciprocating the arc discharger with respect to the inner surface of the graphite mold. At the same time, the silica powder compact is vacuumized through air passages which are open to the inner surface of the graphite mold, and the raw powder is melted and solidified, thereby manufacturing the silica crucible having a thickness of, for example, 10 mm (see FIG. 3B).
In addition, the vitreous silica crucible obtained by the aforementioned method has a double laminated structure constituted by an outer layer composed of high-purity amorphous vitreous silica glass with a bubble content (percentage of bubbles included in vitreous silica per unit volume) of 1 to 10% and a purity of 99.99% or higher and an inner layer composed of high-purity amorphous silica glass with a bubble content of 0.6% or less and a purity of 99.99% or higher, and a ratio in thickness between the inner layer and the outer layer is generally 1:1 to 5 (for example, see FIG. 2).
In addition, a single-crystal silicon ingot is manufactured by, as illustrated in FIG. 4, supplying a high-purity polysilicon mass to a vitreous silica crucible fixed to a graphite support, melting the polysilicon mass by using a heater provided along an outer circumference of the graphite support so as to be converted into a silicon melt, heating the silicon melt to the predetermined temperature in the range of 1,500 to 1,600° C. and maintaining the temperature, while rotating the vitreous silica crucible, in an Ar gas atmosphere under reduced pressure, simultaneously rotating a silicon seed crystal to be dipped into the silicon melt surface, and pulling the silicon seed crystal (JP-A-11-171687).
In addition, in the manufacturing of the single-crystal ingot, as also illustrated in FIG. 4, the silicon melt moves from a lower portion of the single-crystal ingot toward a lower portion of the crucible in the vitreous silica crucible, and flows upward from the lower portion of the crucible along an inner surface of the crucible, by convection flowing toward the lower portion of the single-crystal silicon ingot. Meanwhile, the silicon melt (Si) reacts with the inner surface (SiO2) of the crucible to generate SiO gas. The generated SiO gas moves along with the convection of the silicon melt toward the silicon melt surface. Here, so as not to enable the generated SiO gas at the silicon melt surface to move into the single-crystal ingot under pulling and generate pinhole defects, the pulling condition is controlled to discharge the SiO gas to the pressure-reduced Ar gas atmosphere so as to be removed.
In addition, in order to extend a life-span of the vitreous silica crucible by preventing deformation and distortion of the vitreous silica crucible that may occur during the pulling of the single-crystal silicon ingot, techniques such as providing a crystallization promoter composed of, for example, oxides of alkaline-earth metals, hydroxide, carbonate, or the like, between the inner layer and the outer layer of the vitreous silica crucible (JP-A-2006-124235), or applying the crystallization promoter to an outer circumferential surface of an upper end portion of the opening of the crucible (JP-A-2005 -255488) have been developed. In the techniques, during the melting and molding of the vitreous silica crucible, by the action of the crystallization promoter, an amorphous (glassy) structure is converted into a crystalline structure, thereby increasing the strength of the vitreous silica crucible.
Recently, as single-crystal ingots have been manufactured to have larger diameters, it has become possible for large-diameter single-crystal silicon ingots with diameters of 200 to 300 mm to be manufactured. Correspondingly, large-diameter vitreous silica crucibles with inner diameters of 610 to 810 mm have been needed. As a result, manufacturing large-diameter vitreous silica crucibles with a crystalline structure having high strength has become necessary. In the conventional vitreous silica crucible that obtained high strength by the action of the crystallization promoter to convert the amorphous structure into the crystalline structure, as the pulling time is lengthened due to the increase in diameter of the single-crystal ingots, by the action of the crystallization promoter existing in the crucible, the amorphous structure crystallizes even during pulling, more fine crystal grains are obtained, and crystal grain boundaries further increase. Reaction between the silicon melt (Si) and the inner surface (SiO2) of the crucible actively occurs in the crystal grain boundaries as compared with the amorphous structure. In this aspect, with the increase in the crystal grain boundaries, an amount of the generated SiO gas significantly increases. In addition, the large amount of generated SiO gas cannot be sufficiently discharged to the pressure-reduced Ar gas atmosphere from the silicon melt surface so as to be removed. The residual generated SiO gas that is not removed moves to the lower portion of the single-crystal silicon ingot under pulling along with the flow of the silicon melt and is incorporated into the single-crystal silicon ingot causing pinhole defects.
Therefore, the inventor, according to the above-mentioned aspects, has carried out research related to the vitreous silica crucible used for pulling a large-diameter single-crystal ingot.
As a result, it has been observed that the strength of the vitreous silica crucible during pulling of the single-crystal ingot is determined by the strength of an upper end portion of the opening of the crucible. Therefore, as long as high strength is guaranteed for the upper opening end portion of the crucible, although only the upper opening end portion of the crucible is in a crystalline structure and other portions (practically, below the ingot-pulling start line of the silicon melt surface) are in an amorphous structure, the vitreous silica crucible maintains high strength during the pulling of the single-crystal silicon ingot. Therefore, deformation and distortion of the vitreous silica crucible can be prevented.
As described above with reference to the conventional vitreous silica crucible, when the vitreous silica crucible in which the crystallization promoter exists is used for pulling the single-crystal silicon ingot, the vitreous silica crucible further crystallizes during the pulling of the single-crystal ingot by the action of the crystallization promoter, and fine crystals are obtained. As a result, crystal grain boundaries increase, reaction of Si+SiO2 actively occurs in the crystal grain boundaries, and this promotes generation of pinhole defects in the single-crystal ingot under pulling. However, it has also become apparent that when the crystallization promoter does not exist in the vitreous silica crucible, crystallization (increase in crystal grain boundaries) of the vitreous silica crucible does not proceed during pulling.
Therefore, as illustrated in FIG. 2, in the vitreous silica crucible according to the invention, a ring-shaped cut-off portion corresponding to a portion extended upward from the upper end of the opening of the crucible is provided to the high-purity vitreous silica during molding of the vitreous silica crucible, and a crystallization promoter that is composed of one or more kinds selected from the group consisting of aluminum oxide (hereinafter, represented as Al2O3), calcium oxide (hereinafter, represented as CaO), barium oxide (hereinafter, represented as BaO), calcium carbonate (hereinafter, represented as CaCO3), and barium carbonate (hereinafter, represented as BaCO3), is added to the ring-shaped cut-off portion. When the crystallization promoter to be added accounts for 0.01 to 1 mass % of the total amount of the crystallization promoter and the high-purity vitreous silica to melt and mold the vitreous silica crucible material, the upper opening end portion of the vitreous silica crucible main body becomes a crystalline structure. Here, the results showed that in the portion between the upper opening end of the vitreous silica crucible and the ingot-pulling start line of a silicon melt surface in the step of pulling a single-crystal silicon ingot, a portion corresponding to 40 to 100 volume % from the upper opening end of the crucible is set to be in the crystalline structure, the large-diameter vitreous silica crucibles with inner diameters of 610 to 810 mm obtain high strength without generation of deformation, distortion, and the like upon pulling the single-crystal ingot.
After melting and molding the vitreous silica crucible material, in the vitreous silica crucible main body from which the ring-shaped cut-off portion including the crystallization promoter is removed, the crystallization promoter does not exist. Therefore, the amorphous portions (remaining portions excluding the crystallized portions between the upper opening end of the vitreous silica crucible main body and the ingot-pulling start line of the silicon melt surface) do not further crystallize during the pulling of the single-crystal ingot and remain in the amorphous structure. Accordingly, generation of the SiO gas can be significantly suppressed as compared with the crystalline structure, and correspondingly, generation of pinhole defects in the single-crystal ingot can also be suppressed. This phenomenon also occurred in the large-diameter single-crystal ingot with the diameter of 200 to 300 mm.